In order to provide a continuous supply of ATP, heart cells contain thousands of mitochondria, which may be a source of, and subject to damage by, oxidative stress. We have found that cardiac mitochondria are organized as a network of oscillators, whose degree of coupling and synchronization is influenced by reactive oxygen species (ROS). Under pathological conditions, e.g. ischemia-reperfusion, either an increase in ROS production, or a decrease in the capacity to scavenge ROS, results in the collapse or oscillation of mitochondrial inner membrane potential throughout the cell, and in clusters of cells in the myocardial syncytium. In this way, mitochondrial dysfunction scales to produce organ level heterogeneity that significantly alters the electrophysiological and contractile properties ofthe heart Over the prior award period, we have established that stabilization of mitochondrial inner membrane potential by pharmacological agents targeting mitochondrial benzodiazepine receptors can prevent post-ischemic arrhythmias and decrease ischemia-reperfusion injury and we have suggested that a specific inner membrane anion channel (IMAC) was the primary target of ROS, independent of the classical permeability transition pore (PTP). In the present proposal we seek 1) to identify the key proteins implicated in the mechanism of mitochondrial ROS-induced ROS release using molecular methods and experiments in isolated cells and mitochondria, 2) to define the main biochemical pathways responsible for scavenging ROS in cardiac mitochondria and their impact on the approach to mitochondrial criticality, 3) to elucidate the mechanisms of mitochondrial-to-nuclear communication via the redox status ofthe cell, and 4) to continue to develop and expand our integrated computational models of excitation-contraction coupling, mitochondrial energetics, and ROS-induced ROS release to the tissue level to understand the mitochondrial origin of cardiac arrhythmias and contractile dysfunction in heart disease.